Water insoluble salts such as calcium and magnesium carbonates or silicates or sulfates commonly referred to as limescale, but also barium sulfate, calcium oxalate, calcium phosphate, iron oxide and the like are readily formed in watery solutions when the conditions are right and may each represent particular challenges in relation to their removal.
Limescale or limestone is the hard, off-white, chalky deposit found in kettles, hot-water boilers and the inside of inadequately maintained hot-water central heating systems. It is also often found as a similar deposit on the inner surface of old pipes and other surfaces where “hard water” has evaporated.
These types of limescale differ slightly due to their origins. The type found deposited on the heating elements of water heaters, laundry machines, etc. has a main component of calcium carbonate, precipitated out of the (hot) water. Hard water contains calcium (and often magnesium) bicarbonate and/or similar salts.
Calcium bicarbonate is soluble in water, however at temperatures above 70° C. the soluble bicarbonate is converted to poorly-soluble carbonate, leading to deposits in places where water is heated. Local boiling “hot spots” can also occur when water is heated, resulting in the concentration and deposition of salts from the water. Likewise calcium sulfate is a common component of fouling deposits in industrial heat exchangers, due to its decreased solubility with increasing temperature. Silicate containing laundry and automatic dishwashing products may cause a calcium or magnesium silicate deposit, which is especially difficult to remove (in contrast to calcium carbonate) from glassware.
The type found on air-dried cooking utensils, dripping taps and bathroom tiling consists of calcium carbonate mixed with all the other salts that had been dissolved in the water, prior to evaporation. It can also be found on taps and water reservoirs (such as in the toilet) where hard water has been continually running through and has deposited calcium carbonate.
The presence of limescale presents several problems. Other than being unsightly and harder to clean, limescale can impair the operation of various components or damage them. In kettles, limescale acts as an insulator, impairing heat transfer. Additionally, it can damage the heating element, which overheats due to accruing limescale. Limescale can build up inside tubing, thus reducing water flow and necessitating higher electrical consumption for the circulation pumps, and eventually blocking the tubing. Expresso machine manufacturers recommend to descale the machine (depending on the water hardness) every month or trimester in order to avoid bitter taste development, machine malfunction and slowing down.
Other types of deposits formed by insoluble salts are beerstone and milkstone. Calcium oxalate forms a major component of beerstone, a brownish precipitate that tends to accumulate within vats, barrels and other containers used in the brewing of beer. Beerstone is composed of calcium and magnesium salts and various organic compounds left over from the brewing process; it promotes the growth of unwanted microorganisms that can adversely affect or even ruin the flavor of a batch of beer. Calcium oxalate is also formed during carbonation of raw sugar beet juice before it undergoes crystallization. First, the juice is mixed with hot milk of lime (a suspension of calcium hydroxide in water). This treatment precipitates a number of impurities, including multivalent anions such as sulfate, phosphate, citrate and oxalate, which precipitate as their calcium salts and large organic molecules such as proteins, saponins and pectins, which aggregate in the presence of multivalent cations.
Milkstone is a layer of scale mainly formed by cations like calcium and magnesium originating from both milk and hard water. Besides giving the equipment an unclean appearance, milkstone could harbour and protect micro organisms always present in raw milk and ready to multiply at a high rate. Since milk products are some of the most perishable major foods, cleaning and sanitization in that industry generally require the highest standards. The main part of milk residu is easily removed by rinsing with water. However, the last part comprising the milkstone is often harder to get rid of.
Several methods and products have been developed in order to remove some or all of these different types of deposits by insoluble salts, such as limescale.
Generally, different types of descaling agents are used to remove deposits by insoluble salts. Descaling agents are either acids or complexing agents or both in one (e.g. carboxylic acids). They remove insoluble deposits such as limescale by respectively dissolving the limescale and or complexing its cationic constituents. Acids used as descaling agents can be either mineral acids or organic acids. Below in table 1, the properties of some organic and mineral acids that are used or can be potentially useful for descaling are shown.
TABLE 1Properties of some organic and mineral acids used or potentially useful for descaling.SolubilitySolubilityTrivial namewatercalcium saltphysicalof acid20° C.g/100 mlformSourcingdescalingpKasmellCompatibilitylabeloxalic 14%  0.0007powderPetro1.3/4.3++C, Xnmaleic>40%2.9powderPetro1.9/6.3+−Xi/Xnmalonic>90%No datapowderPetro2.9/5.7+−Xntartaric+−60%   0.04powderFerm 3/4.3++Xifumaric 64%1.4,2%bpowderPetro 3/4.4++Xicitric 60%5%(1.Ca)powderFerm−3.1/4.8/6.4++++Xi0.09%(2-3.Ca)malic>80%  0.8%bpowderPetro+3.4/5.1+−−−Xnformic100%17  liquidPetro++3.8−−−−C, Xnglycolic100% 1.2%powderPetro−−3.9+−−−C, Xnitaconic 9.5%No datapowderFerm3.9/5.1++Xilactic100%7%,3.1%bliquidFerm+3.8++++Xigluconic>50%3,3%bpowderFerm3.9−−Xnsuccinic 7.7%   0.004%bpowderpetro/ferm4.2/5.6++C, Xnglutaric 50%SolublepowderPetro4.3/5.4++XiAcetic100%33.8 liquidpetro/ferm+4.8−−−−C, XnLactideaInsol.aaPowderFerm/++++Xiphosphoric100% 0.03liquidMin++ 2.2/6.8/12.4+−C, Xnsulfamic 29%No datapowderMin++0.1+−+−C, Xn, Nhydrochloric>40%75  liquidMin−9.3  −−C, T, Nsulfuric100%0.3liquidMin−3   +−C, TbLactide is a dimeric ester rather than an acid but readily hydrolyses to lactic acid. Data obtained from presentations by Purac and complemented with various literature data. Properties listed include the solubility in water (pH 7, 20° C.), the solubility of the calcium salt (mono, di, tri-salts,bas % anhydrous at 25° C.), their physical form, descaling effectiveness (Purac data), pKa value(s), smell, overall material compatibility (Purac data) and labeling according to EU legislation.
Table 1 documents among other characteristics the water solubility of di- and tri-salts of polybasic carboxylic acids which tends to be (very) limited as compared to that of monocarboxylic acids, with maleic and glutaric acids as an exception to this apparent rule. No literature data were found regarding the calcium salt of itaconic acid. Whereas the monocalcium salt of citric acid is water soluble (5%), the disalt and trisalts are only sparingly or practically insoluble (0.09 g/l).
The majority of acids commercially used for descaling are mineral acids such as phosphoric, sulfamic, hydrochloric and sulfuric acid (cf table 1). These are however classified as corrosive to the skin and the eyes and as environmentally hazardous or in case of phosphoric acid represent a substantial eutrophication potential. Moreover they tend to be either fuming or cause a pungent smell and their overall material compatibility is limited.
Organic acids have one, two or three carboxyl groups (note the pKa values in table 1) and are usually less aggressive which is why acetic, citric and formic and more recently glycolic and lactic acid found their way to the market.
Organic acids can be sourced from fermentation or from petrochemical synthesis. Citric and lactic acid for example are obtained by fermentation from renewable feedstock (typically molasses). The fact that many of these organic acids suitable for descaling action are renewable is increasingly considered an environmental advantage as illustrated in life cycle analyses. However, some of these organic acids still show disadvantages.
For example, the iron and calcium salts of citric acid are said to be less soluble than those of glycolic acid, so they may precipitate onto the treated surfaces, diminishing cleaning effectiveness of citric acid.
Acetic and formic acid have a pungent smell that is hard to cover with fragrance, which is a serious disadvantage.
Acetic acid, which may be sourced from fermentation or from petrochemical synthesis, is renowned for its corrosivity to copper which leads to the formation of toxic copper acetate (a fungicide) thus rendering acetic and vinegar unsuited for descaling coffee and expresso machines which often have a copper mounting tube for hot water or steam. Acetic acids will thus also be unsuited for all other surfaces comprising cupper.
Furthermore, the descaling activity of many organic acids is quite weak. Many organic acids either show efficiency in fast descaling or in descaling upon prolonged contact, but not both. Moreover a limited number of organic acids is available as a solid. These are huge disadvantages as they put a restriction onto the development of descaling agents that offer an overall better efficiency.
There is a need for a descaling agent which is renewable, and which shows a better efficiency than the existing products.
One specific application of descaling agents is their use in toilet blocks, since toilets often suffer from severe insoluble salt deposits. Traditionally toilet blocks in the past where designed to mask odors and have a slight cleaning effect in the toilet. The two main types manufactured and marketed up until the late 1980's were the so called rim and the in-cistern blocks, applied in the toilet bowl and the water cistern respectively. During the 1990's several new developments have been marketed, with the liquid rim containers coming on the market which has greatly increased the flexibility and number of ingredients available to formulate with, and the solid block formulations have also been expanded with products that have special properties (i.e. lime scale inhibition, bleaching, cleaning efficiency, etc.).
There are different challenges to the formulation and manufacture of toilet and cistern blocks as these are dependent on most of the ingredients being supplied as practically water free chemicals, otherwise they might have a negative influence on the chemical properties of the block as well as the stability and compatibility with other ingredients included. The main manufacturing process for such blocks is by extrusion of a pre-made dry mixture of all the ingredients. A crucial property for the manufacture of solid blocks however has been the extrusion properties of the anionic surfactants and in particular dry LAS (Linear Alkyl Benzene sulfonate). The sodium salt of LAS in dry form is available as a very hygroscopic powder, which means that precaution has to be taken in terms of handling and storage, but it is also this product characteristic that makes it an excellent main ingredient in formulating solid extrude toilet blocks. The hygroscopic nature of LAS ensures that once the final product is exposed to water in the toilet bowl or in the cistern it will create an outer layer or membrane that slows down the overall solubility of the block thereby imparting a controlled release of all the active ingredients in the block (source: Toilet block introductory Leaflet by Unger, 2008). Formulating rim and in-cistern blocks among others implies selecting solubility retarding or “matrix” ingredients with a melting point at or just above the extrusion temperature, which upon cooling will form a homogenous solid block that will gradually and evenly set free its actives over time, typically during several weeks for 50 to several hundreds of flushes, more typically up to 500-800 flushes. Such formulas contain 25-50% LAS (typically 40%), 0-8% fatty alcohol sulfate (mainly C12-14, some C16 in cistern blocks) or 0-5% highly ethoxylated fatty alcohol (e.g. C16-18 with up to 50 mol ethylene oxide), 0-3.5% Coconut monoethanolamide, 1% foam enhancing fatty alcohol ether sulfate, 0.05% paraffin oil, 5-6% fragrance and dyes and sodium sulfate as a filler. Low amounts of acids (e.g. 2% lactic acid or 10-20% citric acid anhydrate) have been incorporated as well as polymers. US2007191245A1 for example describes the use in toilet blocks of polysuccinimide for preventing or dispersing urine scale.
Effervescent toilet descaling tablets form an alternative approach for descaling, targeting fast tablet disintegration (as opposed to toilet blocks) but long contact times (e.g. overnight). They are produced by tabletting and always contain an acid (usually sulfamic or citric acid) for dissolution of the immersed limescale and a carbonate source for the effervescent system (sodium carbonate, bicarbonate, percarbonate, . . . ). Formulating such tablets is all about finding the balance between fast dissolution on one hand and tablet strength and stability on the other. Low moisture content is of paramount importance, especially when the formula contains percarbonate bleach. A typical formula contains 1-2% lauryl sulfoacetate or FAS, 1% FAEO C16-18 8EO, 40-50% citric or sulfamic acid, 20-30% sodium carbonate, some polyethylene glycols, fragrance, dye, and sulfate as a filler. Some formulations additionally contain about 2% percarbonate bleach.
Products dedicated to periodical cleaning and descaling of automatic dishwashing machines usually are based on citric acid and a small amounts of FAEO (e.g. C9-11, 4EO), and additionally may contain some corrosion inhibitor, solvents, PEG, phosphonates, fragrance and dye.
There is a need for a renewable low moisture and stabile descaling agent that can be used in descaling block or tablet formulations and has a better efficiency than the existing descaling agents used in toilet blocks and tabs.
It is an object of the present invention to provide a new descaling agent which is made of renewable material and which shows a better overall efficiency than the descaling agents known from the prior art.
It is also an object of the present invention to find a toilet block comprising a descaling agent, which is made of renewable material, is low moisture and stabile with a better efficiency than the existing descaling agents used in toilet blocks.